Downton Abbey’s first Christmas Special is the two-hour centrepiece of ITV1’s 2011 Christmas Day line-up and a guaranteed ratings triumph.

It serves up a sepia-tinted vision of the English class system, in a frictionless yesteryear where the well-to-do are tended by cap-doffing underlings. Social mobility being roughly where it was a century ago (and Etonians being back in charge), it might almost be a snapshot of modern UK plc. But Downton Abbey – every wobbly plotline of it, every galloping implausibility – has been universally devoured as the purest escapism, in this country, in America and apparently all the way to Outer Mongolia.

Even those who were maddened by the second series’s more risible shortcomings, not to mention the first series’s refusal to tie up a single bow, are clamped in its vice-like grip.

Not that addiction to Downton is, for its viewers, so very time-consuming. So far it’s claimed perhaps a full waking day. For the actors, however, it’s another story altogether. In February they return to Highclere Castle, the castellated Berkshire botch-up that passes for the Crawleys’ ancient abbey, to make a third instalment of the series, and won’t emerge for many moons. It now dominates their lives.

A selection of the cast attended a preview screening of Downton Abbey’s first Christmas Special – the two-hour centrepiece of ITV1’s 2011 Christmas Day line-up and a guaranteed ratings triumph – earlier this week.

It would not be breaking the terms of the draconian embargo that all those in attendance were required to sign to report that they look oddly unfamiliar with their contemporary hairdos, modern threads and unbuttoned body language.

And the public can be quite blunt about the discrepancy. “I had somebody come up to me,” says Siobhan Finneran, who plays the intermittently shrewish lady’s maid O’Brien, “and say, 'Can I just say something? Television does nothing for you.’ I had to take that as a compliment.”

Lesley Nicol (Mrs Patmore, the cook) has endured similar gawps. “A very drunk man came up to me,” she says, “and went, 'Oh my god you look 40 years younger.’ Which would make me 10.”

Such is the power exerted by the story that it’s hard to avoid a double take when you find that, out of character, Finneran is not a sourpuss but waspishly funny, Brendan Coyle (Mr Bates, the valet) is a hoot, and there is a lot less grey starch in Phyllis Logan than in the part she plays, the housekeeper Mrs Hughes.

Entering the gilded cage that is the Downton set requires its actors to playing emissaries from the pre-Freudian age on whom manners and costume have a restraining influence. Do they enjoy having to button up and keep a lid on?

“It’s strangely liberating to have those constraints on you,” says Logan. “It’s challenging to know that you’re got to get a certain amount across in a more subtle way than by giving it full welly.”

Coyle has spent two series practising the rarefied art of emotional constipation. “The stoicism is part of the appeal,” he says. “Now we’re all encouraged to express ourselves. People weren’t so much then, especially in the lower classes. That doesn’t mean that you don’t convey high emotion. In the Christmas special the stakes are very very high for almost all the characters.”

Even Rob James-Collier seems notably unvisited by the traits of Thomas the dastardly footman. Not that that stops viewers from confusing him with his character. “I’ll go into a Post Office and get booed,” he says. “I’ll get a bit of stick from a man in a coffee shop but then he’ll pay for my latte. It’s swings and roundabouts.”

The careers of the cast have become defined by Fellowes’s posh romp. Thirty years from now they will doubtless be summoned to reunions by Bafta much as the cast of Upstairs, Downstairs have been. You wonder if there’s any wriggle room for them. If any of the cast could ask for a present from their lord and scriptwriting master Julian Fellowes, what would it be?

“My basic hope for Daisy is a new dress,” says Sophie McShera, who plays the kitchen under-scrubber in the same characterless rags week in week out. “I want a bloke,” says Lesley Nicol plaintively, “It’s a bit insane because Mrs Patmore looks like the back of a bus. I did say to Julian, 'I know it sounds crackers’ and he said, 'Never say never.’”

Some of the cast entertain more highfalutin fantasies. “Maybe I’ll take up flying now that I can drive,” suggests Laura Carmichael, who plays the perennially overlooked middle sister Lady Edith. “There’s a lot of costume envy,” says Finneran. “The girls upstairs wear amazing costumes. We’re usually neck to floor in black. I’m quite hoping O’Brien discovers false eyelashes and a bit of red lippy.”

The last time we saw them, Lady Mary and Matthew were condemned to yet more years of agonised romantic postponement. Mr Bates looked destined to swing and even his Lordship was sinking into a midlife crisis. Only Lady Sybil, who has won her father’s permission to marry the uppity Irish chauffeur, can be guaranteed a dose of the happiness that Fellowes doles out so parsimoniously.

By rights we ought to be spending Christmas with her, but as she’s already moved to Ireland, the spotlight falls back on Mary and Matthew.And there’s no doubt what Downton devotees will be wishing for this Christmas.